What is the difference between an induction furnace and an induction ccoktop

Could some electronics guru shed some light on
the following ? What is the difference between
an induction furnace and a cooktop, although
the underlying scheme is the same ? For example,
a few years ago I saw some colleagues of mine
use an induction furnace (a small one) to melt
metal alloy powders for rapid prototyping. The
resonant frequency was something like 40 - 50
KHz, and the job was done in 5 - 10 minutes.
Such a furnace would obviously not work for a
cooktop. So, what would be the necessary
modifications.
..

 

Extensive. Cooktops are low-Q coils with high inductance, in other words,
they're made like transformers. The control circuit is also useless
because it's made to sense if a pot is on top. If you tried, say, using
one with an average work coil, made of a few turns of copper tubing, at
best you'll simply get no power out; at worst you'll start a fire from the
exploding transistors.

Such a furnace BTW would certainly work for a cooktop, you just have to
turn it down a little so you cook the food instead of melting it.

Tim

 

Hey, If I might hijack this thread. I've got an electric stove with
spirals of resistive heating elements that get hot. The knob on front
controls the amount of current (or something.) What does the knob
control? How is the current/ voltage varied? Is it a traic or some
such thing?

Thanks,
George H.

 

From my understanding:
The induction furnace has a 'built in' cauldron, a carbon sink, you
put the metal stuff into. The resonant frequency heats the sink, the
sink heats the metal.
The stove top is more like the primary side of a broadband current
transformer. Sitting there looking for something to put the current
into. YOU supply the pots and pans.

Be careful of the induction furnace. It will heat a metal washer, melt
it, and burn through any metal mounting hardware. Make everything
using nylon type bolts and hardware.Even a hole drilled into metal
will heat, so use ABS or such. Out to at least 3 diameters away from
the induction furnace coils.

 

Grin, yeah the stove is not quite that old. I sorta know how to deal
with a few watts of power. (In my inefficient 'class A' style.)
Hundreds / thousands of watts is harder... I first think of a Variac,
but that’s not in my stove either.

George H.

 

The obvious modification is to reduce the power? And a cooktop is not a furnace? The same principle of using induced eddy currents to heat the ferromagnetic cooking container which then heats the food applies?

 

The old ones are closer to your car's turn signal blinker. A heater and
bi-metal strip, with the knob adjusting the tension of the bi-metal
strip with a cam. Slow PWM.


Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

 

Sounds like you have an electric radiant cooktop:
http://www.ehow.com/list_7393291_differences-between-radiant-induction-cooktops.html
Radiant cooktops do not work by induction. And they're not fussy about the cookware. You can't use the aluminum non-stick junk with an induction cooktop, but obviously they're good enough for the radiant.

You just can't beat the bimetallic element where the knob adjustment presses on the bimetal adding a bias to the amount of heat required to trip it. This is because all these controls have time constants in the 10s of seconds..

 

My stove is about 35 years old. It has spiral resistance elements.
The controller for an element can be heard to click on and off from
time to time while operating. I experimented with it this morning,
removing the heating element. There is still a clicking on and off,
coming from the control knob, and it will click on and off at
different points on the dial, depending how long it has been at a
particular setting.
I infer that there is a thermostat with its own heater within the
control, since it doesn't depend on having a heating element
installed.That thermostat must control its own heater as well as the
stove heating element.

 

OK thanks Jon. I've never noticed the AM radio interference that I
get with light traic dimmers.

Maybe just cheaper?

George H.

 

Yeah! I hear it clicking.

I'm cooking up a big pot of spaghetti sauce.
It's the boys annual fishing trip to Oneida lake this weekend.
I'm testing out my theory that you can never put
too much onion or garlic in the sauce.
(Three cloves, the kitchen reeks! tastes OK, should be great by Friday
night)

George H.

 

It's called an "infinite heat control" and is essentially an
electromechanical PWM. There's an internal heater, typically in series
with the external load.


Best regards,
Spehro Pefhany

 

All commercial cooktops AKA stoves use plain resistive heating
elements which are good from DC to 400Hz (ie: anywhere in the world if
voltage matches).
An induction furnace system WOULD work in a cooktop configuration.
Fer eggzample, the RF generator drives the induction loop, which is
then loaded with the cooking pot.
Might not be as efficient as a properly configured (industrial)
furnace at the same power, but it would work OK.

 

Overall efficiency should be excellent -- they use a lot of ferrite and
litz wire in the coil, no need for water cooled copper pipes.

Tim

 

Yep, thermal oscillator with a duty cycle control...

And some of the really old ones used a big element, and small element,
and connected both (in parallel) to 240V for high heat, and connected one
or both to 120V and/or 240V to achieve intermediate heat outputs. 120V
to both elements in series would be 'low heat'.

 

That's completely untrue. Bimetal element controls dominate the market. There are a few high end cooktops that regulate the temperature of the element, versus the power, and those may use an electronic control, but the vast vast majority do not.

 

You did say 3 cloves of garlic, not three bulbs? My cooking would be more
like 3 bulbs, about 20 cloves, maybe a lot more depending on what i made.
Assuming about 8 quarts of product.

?-)